1. Field of the Invention
The present invention relates to a thermal transfer image-receiving sheet which, in use, is laminated on a thermal transfer sheet and receives a colorant thermally transferred from the thermal transfer sheet by means of a thermal head as a device to form an image, and more particularly to a thermal transfer image-receiving sheet which can provide a full-color, high-density recorded image, when used in a thermal transfer recording system using a sublimable dye as a colorant, and has a high glossiness.
2. Background Art
Various thermal transfer recording systems are known in the art, and one of them is a sublimation transfer recording system in which a sublimable dye as a colorant is transferred from a thermal transfer sheet to an image-receiving sheet by means of a thermal head capable of generating heat in response to recording signals, thereby forming an image. In this recording system, since a dye is used as the colorant and the gradation of the density is possible, a very sharp image can be formed and, at the same time, the color reproduction and tone reproduction of half tone are excellent, making it possible to form an image having a quality comparable to that formed by silver salt photography.
By virtue of the above excellent performance and the development of various hardware and software associated with multi-media, the sublimation transfer recording system has rapidly increased the market in a full-color hard copy system for computer graphics, static images through satellite communication, digital images represented by CD-ROM, and analog images such as video.
Specific applications of the image-receiving sheet in the dye sublimation transfer recording system are various, and representative examples thereof include proof printing, output of various images, output of a design, such as CAD/CAM, output applications for various medical instruments for analysis, such as CT scan, output applications for measuring equipment, alternatives for instant photography, output of photograph of a face to identification (ID) cards, credit cards, and other cards, and applications in composite photographs and pictures for keepsake in amusement facilities such as amusement parks, museums, aquariums, and the like.
The thermal transfer image-receiving sheet for sublimation transfer used in the above various applications (hereinafter referred to simply as "thermal transfer image-receiving sheet" or "image-receiving sheet") generally comprises a substrate and a color-receptive layer formed thereon. It is needless to say that what is first required of the image-receiving sheet is high sensitivity in printing and good stability against curling associated with printing.
Further, the diversification of applications of the image-receiving sheet has expanded the market and has led to an ever-increasing demand for color reproduction in halftone, a sharp image free from an uneven density, and high glossiness and smoothness which can impart high-grade feeling to the appearance and improved aesthetic properties.
Under these circumstances, for example, Japanese Patent Laid-Open No. 122991/1990 describes a method for imparting high glossiness and smoothness, wherein a plastic film is put on the surface of a colorant-receptive layer and hot lamination is carried out by heating under pressure. This method, however, necessitates providing additional steps and, further, preparing a plastic film, resulting in remarkably increased production cost.
Japanese Patent Application Nos. 87390/1987, 278087/1987, and 246153/1993 teach the provision of a thermoplastic surface layer (hereinafter referred to as "skin layer"), substantially free from any inorganic fine powder or microvoids, on the outermost surface of a plastic film or synthetic paper used as a substrate or a part of the substrate for an image-receiving sheet.
These methods can certainly provide a high glossiness. The glossiness, however, is excessively high, and, consequently, fine scratches unavoidably created in the production of an image-receiving sheet become very clearly visible, resulting in lowered commercial value due to poor appearance. The fine scratches are not causative of the occurrence of dropout or uneven print density in the printing. They, however, are very clearly visible when the surface has a very high glossiness and is close to a specular surface.